The 15W just means it will be thicker when cold, which is when you don't want it to be thicker, although it may help with cold engine rattles.
It might also help to close up ring gaps when cold and reduce oil burning then.
Closing the gap between the cold and hot viscosity numbers is a good thing because it should mean less VI modifiers have been used, so the oil may stay in grade for longer.
There will however be less flow when cold, and/or it will bypass the filter for longer, both of which are bad things.